1. Field of invention
The present invention relates to a feed arrangement for the inked ribbon of a high-speed printer for office machines such as accounting machines, data terminals and teleprinters, which comprises a pair of spools for the ribbon.
2. Description of the prior art
In high-speed printers for accounting machines, teleprinters, and similar machines, it is important that the feed of the ribbon should take place in a reliable manner and by means of mechanisms which are not very bulky.
In these printers, an inked ribbon of considerable length is moreover used, so that the diameter of the feed spool varies considerably from the time when the spool is full to when the ribbon is fully unwound. The pull to which the ribbon is subjected consequently varies considerably in the course of the unwinding of the ribbon from the feed spool to the take-up spool. This therefore gives rise to an unacceptable slackening of the ribbon or to an excessive stress thereon, with the danger of high rates of wear or of tearing.
An arrangement is known in which each feed and take-up spool is associated with an electric motor of the type having an axially movable rotor and is mounted on the frame of the machine. When these motors are supplied, they shift the rotor axially simultaneously with the starting of the rotation in such manner as to bring a driving pinion keyed on the rotor itself into engagement with a driven gear connected to the corresponding spool. In this arrangement, with the pinion disengaged, the slidable rotor actuates, through a spring, a shoe which brakes the driven gear. When a motor is supplied, the corresponding shoe is raised and the take-up spool is freely rotated by the motor itself. The other motor, on the other hand, is inhibited, as a result of which the corresponding shoe co-operates with the corresponding driven gear, thus checking the unwinding of the ribbon on the feed spool.
This arrangement has the disadvantage that, since the feed spool is checked or braked with a constant resisting torque, the corresponding resisting force applied to the ribbon is variable and is all the greater the smaller the winding diameter of the ribbon on the spool.
An arrangement for feeding an inked typewriter ribbon is moreover known in which the ribbon passes from a feed spool to a take-up spool. The feed spool is freely rotatable on the carriage bearing the typing head together with the take-up spool and is kept locked by a ribbon tightener pulled by a spring, with a predetermined loop of the ribbon being formed by the said ribbon tightener. With the feed spool stationary, the ribbon passes from the loop to the take-up spool. When the loop reches a minimum, the ribbon tightener releases the feed spool, which reforms the loop, bringing it back to its predetermined value and locking the feed spool again. In this arrangement, when the ribbon is transferred from the loop to the take-up spool, the pull on the ribbon remains sufficiently constant, but when the ribbon tightener releases the feed spool the pull decreases rapidly, therefore only partially solving the disadvantages hereinbefore described.